1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or printer provided with a function to form images on recording material such as sheets.
2. Description of the Related Art
An image forming apparatus, which performs image forming operations using precision techniques, contains components which will be affected if overheating occurs in the apparatus as a result of continuous operation. Above all, elements which particularly require consideration of impacts are imaging sections (image forming sections, process cartridges, toner cartridges and image forming units) involved in formation of toner images.
The imaging section includes elements, such as a developing blade and cleaning blade, which produce heat as a result of rubbing friction, where the developing blade regulates an amount of toner supply to a developing roller while the cleaning blade recovers untransferred toner remaining on a photosensitive drum after a transfer process. Since the toner has the property of being melted by heat, unless these elements are kept below a predetermined temperature, physical properties of the toner are changed by temperature rises, and consequently lumps of toner could get fused to a blade tip, drum surface, or the like. This phenomenon obstructs an imaging process and surfaces in the form of image defects.
A similar problem is generally encountered by those areas of the imaging sections which are concerned with storage and supply of toner, and unless a drive section, fixing device and electrical board of the image forming apparatus are prevented from overheating including thermal influence of the surroundings, it is difficult to guarantee image quality throughout the service life of the cartridges.
Monochrome products which have only one imaging section are relatively free from spatial constraints around the imaging section, flexible in forming cooling air flow paths, and thus able to avoid the above problem relatively easily.
In contrast, color products in which multiple imaging sections are arranged side by side are subject to very strong constraints on formation of air flow paths as follows if it is assumed that the imaging sections 70 are rectangular parallelepipeds as shown in FIG. 7. That is, of the six faces surrounding each imaging section, five faces are closed: two faces (71, 72) are closed by adjacent imaging sections, one face (73) is closed by an intermediate transfer belt, one face (74) is closed by a laser scanner, and one face (75) is closed by a drive section which drives the imaging section. Thus, there are very strong constraints on formation of air flow paths.
Recently, along with downsizing of image forming apparatus, spatial density of parts and units which make up the apparatus has been growing, resulting in still severe spatial constraints. Design concepts of interior overheat prevention are roughly classified into two types. One of the types consists in “providing sufficient interior cooling performance to ensure that safe temperature will not be exceeded under any conditions of continuous operation in guaranteed operating environment.” The other type consists in “detecting interior temperature conditions by means of an interior temperature detection unit and switching to a safe operation mode intended to prevent further temperature rises, before safe temperature is exceeded.” The former design concept will be adopted when sufficient cooling capacity is available for the image forming apparatus. Otherwise the latter design concept will be adopted.
Recent color products are subject to very strong constraints on formation of air flow paths as described above while printing speed which affects heat generation of a drive section and rubbing unit are increasing gradually, making it difficult to secure sufficient cooling capacity for the amount of heat generation. Consequently, the latter method is often selected. Even when the latter design concept is selected, it is very important as product performance to be able to prevent reductions in throughput (productivity) of image formation after switching to the safe operation mode, and thus there is demand to maximize equipment cooling performance.
Under the constraints related to an issue of interior temperature rises in color products, conventionally the imaging sections are cooled using techniques largely classified into two types as described below. One of the types involves forming a flow of air in one direction along a photosensitive drum 85 in an imaging section 80 as shown in FIG. 8. Examples include a technique disclosed in Japanese Patent Application Laid-Open No. 2008-268528. The technique involves supplying cooling air from one longitudinal end of the imaging section using a fan and venting air from another end using another fan. Examples of the other type include a technique disclosed in Japanese Patent Application Laid-Open No. 2005-173335. The technique involves directing cooling air at heat dissipating units installed at opposite ends of the imaging section and thereby releasing heat from the opposite ends.
However, conventional techniques have low cooling efficiency for the imaging sections, and thus have a problem in that it is difficult to satisfy cooling performance required of the products or that a large number of fans are required in order to complement the low cooling efficiency with an air flow volume. This will be described below concretely.
With the former of the conventional configurations (cooling in one direction along the drum), main causes of the low cooling efficiency are the following two factors (see FIG. 8).                Reductions in air flow velocity and air flow volume on the downstream side of the air flow path        Increases in temperature of the cooling air on the downstream side of the air flow path        
There are many open spaces in the imaging section, including a space for laser irradiation from a laser scanner 84 and a space for abutment and separation of a developing device, and a closed air flow path cannot be formed. Consequently, cooling air A supplied to one end of the imaging section 80 is diffused into the apparatus through the open spaces, and a clean flow cannot be formed in an entire area of the photosensitive drum 85. Also, when a drive unit 81 of the imaging section is located on the downstream side of the air flow path, it is difficult to secure an ideal exhaust route due to spatial constraints, and consequently the drive unit 81 generates such flow path resistance as to facilitate diffusion of the cooling air. Due to the influence of the above factors, the air flow velocity and air flow volume reduces downstream along the air flow path.
To ease this problem, a technique is known which reduces diffusion of air flow by installing a fan on the exhaust side as well as on the intake side, such as described in an embodiment disclosed in Japanese Patent Application Laid-Open No. 2005-173335. However, the fan on the exhaust side draws in heat from areas other than the imaging sections as well, making it difficult to achieve an intended cooling effect.
Also, the cooling air rises in temperature gradually in the course of removing heat from the imaging sections. With the configuration which uses a flow of air in one direction for cooling, the temperature of the cooling air increases downstream, reducing the cooling effect, and a large temperature distribution gradient occurs over an axial direction of the imaging sections. A maximum difference in the temperature distribution corresponds to a temperature difference (front-rear temperature difference) between frames 82 and 83 of the image forming apparatus. Generally, the imaging sections are positioned on the frames 82 and 83 of the image forming apparatus at both ends in an axial direction of the drums. The front-rear temperature difference between the frames results in a difference in amount of thermal expansion in positioning portions of the imaging sections, causing front-back difference to be produced in spacing between the positioning portions of adjacent imaging sections. The front-back difference causes a “problem of color misregistration” to output images. As shown in FIG. 8, an intermediate transfer belt unit 86, primary transfer roller 87 and intermediate transfer belt 88 are provided between the frames 82 and 83.
With the latter of the conventional configurations (heat dissipation from opposite ends of the imaging section), again it is difficult to increase the cooling efficiency for the following two reasons.                It is geometrically difficult to increase heat transfer effect of elements in the imaging section.        It is difficult to provide sufficient heat dissipating sections at the opposite ends of the imaging section.        
Generally, elements of the imaging section have “elongated shapes” whose cross-sectional areas are very small compared to longitudinal dimensions. Consequently, the heat generated at the longitudinal center is not transmitted efficiently to both ends. Thus, it is difficult to achieve cooling effect by means of heat dissipation. This tendency becomes more pronounced with increases in the media width supported by the image forming apparatus, making it difficult to dissipate heat from both ends.
Also, although it is necessary to install heat dissipating members which have surface areas contributory to heat dissipation in the heat dissipating sections, it is not easy to install heat dissipating members of a sufficient size in the imaging section where a large number of elements are arranged densely. In particular, a large number of drive couplings and electrical contacts are laid out on the side of an interface between the imaging section and drive unit, creating very strong spatial constraints and making it difficult to supply cooling air to the interface at the same time.
As described above, since the conventional techniques are subject to strong constraints in increasing cooling efficiency, there is demand for a cooling technique which can solve this problem.